External Recipients as DL members

Well you cannot add them directly, you need a matching mail user/mail contact object. So just look for the relevant recipient types and match them against a domain. For example

Get-Recipient | ? {$_.EmailAddresses -like "*gmail.com*"}

Or for a given DG:

Get-DistributionGroupMember DG | ? {$_.EmailAddresses -like "*outlook.com*"}